The knowledge of God is often talked about in purely intellectual terms. Aquinas most definitely thinks that some knowledge of God is possible through rational reflection, as he expounds on in his notorious “Five Ways” whereby he offers several rational methods to argue for the existence of God. However, the Johannine tradition challenges intellectual renderings of our knowledge of the specifically Christian God:

“Beloved, let us love one another, because love is from God; everyone who loves is born of God and knows God. Whoever does not love does not know God, for God is love” (1 John 4:7-8).

“So we have known and believe the love that God has for us. God is love, and those who abide in love abide in God, and God abides in them” (1 John 4:16).

God is love. Picking up on this, Augustine writes in Book VIII of De Trinitate that it is by love that we arrive at the knowledge of God:

No other thing, then, is chiefly to be regarded in this inquiry, which we make concerning the Trinity and concerning knowing God, except what is true love, nay, rather what is love. . . For as there are two commandments on which hang all the Law and the prophets, love of God and love of our neighbor . . . because he who loves God must both needs do what God has commanded, and loves Him just in such proportion as he does so; therefore he must needs also love his neighbor, because God has commanded it . . . the Law and the prophets hang on both precepts. But this, too, is because he who loves his neighbor must needs also love above all else love itself. But “God is love; and he that dwells in love, dwells in God.” Therefore he must needs above all else love God” (De. Trin, Bk. VIII, ch. 7, n. 10).

In other words, we come to know God when we love because the love by which we love is God’s very self. We need not look for God in nature, nor in argumentation, nor even in Scripture. True knowledge of God is derived rather from true love. When we love, we come to know God because it is the very presence of God in our heart which makes love possible.

This Augustinian formula became central in the 12th century in Peter Lombard’s definition of grace as found in Book I of the Sentences. It is the Holy Spirit, Lombard tells us, that is the love by which we love God and neighbor:

It has been said above and it has been shown by sacred authorities, that the Holy Spirit is the love of the Father and the Son by which they love each other and us. It must be added to this that the very same Holy Spirit is the love or charity by which we love God and neighbor. When this charity is in us, so that it makes us love God and neighbor, then the Holy Spirit is said to be sent or given to us; and whoever loves the very love by which he loves his neighbor, in that very thing loves God, because that very love is God, that is, the Holy Spirit (Bk. I, Dist. 17, v. 2).

In other words, for Lombard, Christian love (charity) is the indwelling of the Holy Spirit, so when we love, it is actually the Holy Spirit working in us that is loving with the same love that exists between God the Father and the Son.

Now, Aquinas will openly disagree with Lombard on this point. For Aquinas, charity is not the indwelling of the Holy Spirit, but rather a habit created in the will by the Holy Spirit. For Aquinas, it is the human will that loves by virtue of the fact that the Holy Spirit renders the will capable of loving through the supernatural habit of charity:

it is evident that the act of charity surpasses the nature of the power of the will, so that, therefore, unless some form be superadded to the natural power, inclining it to the act of love, this same act would be less perfect than the natural acts and the acts of the other powers; nor would it be easy and pleasurable to perform. And this is evidently untrue, since no virtue has such a strong inclination to its act as charity has, nor does any virtue perform its act with so great pleasure. Therefore it is most necessary that, for us to perform the act of charity, there should be in us some habitual form superadded to the natural power, inclining that power to the act of charity, and causing it to act with ease and pleasure (II-II, q. 23, a. 2).

Despite this very significant disagreement between Lombard and Aquinas about what charity is, they are in agreement that love (charity) is made possible by an intense intimacy between God and humans. For Lombard, this intimacy is defined in terms of the indwelling of the Holy Spirit; for Aquinas, it is expressed in terms of friendship and a communication of God’s happiness to us (II-II, q. 23, a. 1). For both, hearkening back to the Augustinian position, love is a critically important way in which we come to know God.

Ultimately, Augustine’s, Lombard’s, and Aquinas’s positions on love are metaphysical in nature. They are not saying that we consciously choose to know and love God when we love our neighbor. They are all trying to characterize the essence of human love in light of the fact that God has revealed Godself as love. But for all three, metaphysical reflection on the nature of love can lead us into a more conscious understanding of the God who makes love possible. As Augustine writes,

Let no one say, I do not know what I love. Let him love his brother, and he will love the same love. For he knows the love with which he loves, more than the brother whom he loves. So now he can know God more than he knows his brother: clearly known more, because more present; known more, because more within him; known more, because more certain. Embrace the love of God, and by love embrace God (De Trin Bk. VIII, ch. 8, v. 12).